MECHANISM OF BETA-HYDRIDE ELIMINATION IN ADSORBED ALKOXIDES

The fluorinated ethanols FnCH3-nCH2OH (n = 0-3) form fluorinated ethoxides when adsorbed on oxidized Cu(111), Cu(110), Cu(100), and Ag(110) surfaces. The fluorinated ethoxides decompose at elevated temperatures by beta-hydride elimination to yield fluorinated acetaldehydes. Inclusion of fluorine in the methyl group systematically lowers the rates of beta-hydride elimination, increasing decomposition temperatures by as much as 150 K. Measurements of the equilibrium constants between adsorbed ethoxides and of their decomposition kinetics on the Cu(111) surface serve to quantify the effects of fluorine on both the heats of dissociative adsorption of the ethanols and the activation barrier for beta-hydride elimination. Fluorination has little effect (approximately 2 kcal/mol) on the heat of formation of the ethoxides but has a much larger (approximately 15 kcal/mol) destabilizing effect on the transition state for beta-hydride elimination. This results from an electronic effect in which the trifluoromethyl group in the ethoxide destabilizes a transition state of the form C(delta+)...H(delta-) having net positive charge on the beta-carbon atom.